A key actuating system (1) of a keyboard musical instrument is provided which helps quickly push the keys by reducing reaction force with respect to force of pushing the keys during manual operation, and which supports a person having less strength to play the keyboard musical instrument. The key actuating system for a keyboard musical instrument generates a sound when a key (3) is pushed including: a pressure detection sensor (11) detecting a pushing pressure on the key; a status detection sensor (13) detecting a movement status of the key: an actuator (15) actuating the key in a pushing direction of the key; and a control portion (19), when the pressure detection sensor detects the pushing pressure and the status detection sensor detects motion of the key, controls operations of the actuator in order to maintain the detected pressure by the pressure detection sensor at a pressure threshold which is in a range larger than 0 and smaller than a pushing pressure on the key which is necessary for making a sound.